Ultrasonic transducer

ABSTRACT

This invention is basically, an ultrasonic transducer that can be configured with good reproducibility, that includes an acoustic impedance matching member that gradually changes the acoustic impedance, and which is characterized by 
     An acoustic impedance matching member(2) configured with multiple unit layers (21, 22, 23, . . . 2n), each layer being thinner than a quarter-wavelength of an ultrasonic wave, and 
     each unit layer having a laiminated structure make up of heavy metal(52) and plastic(51) layers with a thickness ratio set between them to change gradually from the unit layer close to the ultrasonic oscillator to the unit layer close to the subject.

(TECHNICAL FIELD)

This invention is basically an improved broadband ultrasonic transducerdesigned for medical or industrial applications. More specifically, itis an ultrasonic transducer with better acoustic impedance matchingbetween an ultrasonic oscillator and the surface of a subject.

(BACKGROUND ART)

FIG. 6 is a sectional view of a conventional ultrasonic transducerconfiguration (prior art). In the figure, number 1 indicates anultrasonic oscillator, 2 indicates an acoustic impedance matching memberinstalled in front, 3 indicates an acoustic damper, and 4 indicates thesubject.

Conventionally, a member having a thickness equivalent to a single orthree-layer quarter-wavelength plate is used for acoustic impedancematching layer 2. The energy transmission efficiency and bandwidth areboth improved when the number of quarter wavelength plates and theacoustic impedance of the material are properly selected. It is alsoknown, however, that an acoustic impedance matching layer consisting ofn layer of a quarter-wavelength plate fails to obtain a major additiveeffect as compared to when n=2 if n is made n>or=3.

Therefore, an improved version of acoustic impedance matching layer 2 isproposed (as noted in published Patent No. Showa 58-18095) thatsuccessively changes the acoustic impedance between the respectivelayers in contact with ultrasonic oscillator 1 and the surface of thesubject. However, it is not easy to actually. produce a compositematerial having a gradually changing acoustic impedance, or if produced,a lack of reproducibility poses a problem.

(DISCLOSURE OF THE INVENTION)

The object of this invention is to provide a ultrasonic transducer thatbetter matches the ultrasonic oscillator with a subject by providing anacoustic matching layer of good reproducibility that gradually changesacoustic impedance.

(METHOD OF SOLVING THE PROBLEM)

To use our invention in the preferred application mode, we employed anultrasonic transducer, consisting of an ultrasonic oscillator and animpedance matching member installed on the ultrasonic radiation surface,characterized by the impedance matching member having a multilayerconfiguration of unit layers, each thinner than a quarter-wavelength.Each unit layer also has a laminated structure consisting of heavy metaland plastic layers with their layer thickness factor being set toincrease gradually in order from the unit layer in contact with theultrasonic oscillator to the other unit layer in contact with thesubject.

(BRIEF DESCRIPTIONS OF DRAWINGS)

FIG. 1 is the schematic sectional view of the preferred application modeof the ultrasonic transducer based on this invention.

FIGS. 2 and 3 are sectional views of individual unit layers that make upthe acoustic impedance matching member shown in FIG. 1.

FIGS. 4 and 5 show the acoustic equivalent circuit of the acousticimpedance matching member shown in FIG. 1.

FIG. 6 shows the schematic sectional view of a conventional ultrasonictransducer (prior art).

(BEST MODE FOR CARRYING OUT THE INVENTION)

An example of the preferred application mode of this invention isdescribed as follows in detail by referring to the drawings.

In FIG. 1, number 1 indicates an ultrasonic oscillator consisting of PZTand other components, 2 indicates an acoustic impedance matching member(that characterizes this invention) installed in front of ultrasonicoscillator 1 (ultrasonic wave radiation surface), 3 indicates anacoustic damper installed on back of ultrasonic oscillator 1, and 4indicates the subject. Acoustic impedance matching member 2 consists ofthe 21, 22, . . . 2n laminated unit layers, each of which is thinnerthan a quarter-wavelength (for example, approx. lambda/10 to 20) andlaminated to each other using a normal application of adhesion or heatadhesion. FIGS. 2 and 3 are sectional views showing more detailedconfiguration of each unit layer that makes up the acoustic impedancematching member. FIG. 2 shows the layers close to the end in contactwith ultrasonic oscillator 1, and FIG. 3 shows the layers close to theend in contact with subject 4. In these figures, 51 indicates a plasticlayer consisting of mylar, polyester, polyolefine, and other elements.The number 52 indicates a heavy metal layer (deposited on top of plasticfilm layer 51) that consists of, for example, copper, chromium, nickel,iron, cobalt, and other metals and their oxides, nitrides, and otherheavy substances (atoms or particles). The thickness factor of heavymetal layer 52 against plastic layer 51 is set to change in order fromthe unit layer close to the end in contact with ultrasonic oscillator 1to the unit layer close to the other end in contact with subject 4. Inother words, thickness t₁, t₂, . . . t_(n) of heavy metal layer 52 (inthe unit layer) is structured to decrease as it moves away fromultrasonic oscillator 1 (t₁ >t₂ >t₃ > . . . >t_(n-1) >t_(n) =0) when thesubject is a human body. Conversely, the thickness of plastic film layer51 is configured in the opposite way. Layer 2n in contact with subject 4is only configured with plastic film layer 51.

Now, the function of acoustic impedance matching member 2 (with theabove-described configuration) is explained as follows by referring toFIGS. 4 and 5.

FIGS. 4 and 5 show the mechanical and electrical representations of theacoustic equivalent circuit of acoustic impedance matching member 2.Mechanically, the equivalent circuit is expressed by masses m1, m2, m3,. . . and the springs (compliances) B1, B2, B3, . . . in seriesconnection. Electrically, it shows a filter circuit consisting ofinductance L1, L2, L3, . . . and capacitors C1, C2, C3, . . . . Here,masses m1, m2, m3, . . . and inductance L1, L2, L3, . . . are part ofheavy metal 52, and the springs (compliance) B1, B2, B3, . . . andcapacitors C1, C2, C3, . . . are part of plastic film layer 51. In themechanical equivalent circuit, the mass and spring have the followingrelationship: m1>m2>m3 . . . >mn and B1<B2<B3 . . . <Bn. In theelectrical equivalent circuit, the inductance and capacitor have thefollowing relationship: L1>L2>L3 . . . >Ln and C1<C2<C3 . . . <Cn.Because the characteristic impedance (acoustic impedance) of a systemhaving equivalent circuits is determined by the ratio between mass m andcompliance B or between inductance L and capacitor C, acoustic impedanceZ₀ of the matching member 2 increases toward ultrasonic oscillator 1 ordecreases toward subject 4. By combining ultrasonic oscillator 1 (whichhas a large acoustic impedance) with the subject (which has a smalleracoustic impedance), the acoustic impedance changes gradually. If theacoustic impedance of subject 4 is greater than that of ultrasonicoscillator 1, the acoustic impedance of the matching member is changedto increase gradually from ultrasonic oscillator 1 toward subject 4.

Individual unit layers that make up the acoustic impedance matchingmember can also be realized by depositing a heavy metal layer on top ofthe plastic layer by evaporation, sputtering or adhering the plastic andheavy metal layers after stratification. The acoustic impedance of theindividual unit layers is determined by the thickness ratio betweenheavy metal layer 52 and plastic film layer 51.

Because it is easy to correctly determine each layer thickness, thematching member with acoustic impedance that changes according to therequired mode from the side of ultrasonic oscillator 1 toward subject 4can be configured with good reproducibility. Moreover, althoughultrasonic oscillator 1 (in FIG. 1) is shown in a single plateconstruction, this construction can include multiple ultrasonicoscillators arranged in a one-dimensional or two-dimensional array.

We have described the preferred application mode for this invention.This invention may be applied very easily in other specific forms bypersons possessing the necessary technical knowledge without departingfrom the spirit or essential characteristics of the following claims.

I claim:
 1. An ultrasonic transducer comprising an ultrasonic oscillatordevice and an acoustic impedance matching member useable in contact withan ultrasonic wave radiating surface, wherein said acoustic impedancematching member consists ofa plurality of layers, each layer being of athickness less that one-quarter wavelength of an ultrasonic wave andconsisting of a sublayer of plastic material and in contact therewith asublayer of metallic material and arranged so that the sublayer ofplastic material alternates with the sublayer of metallic material,wherein the ratio of thickness of the sublayer of plastic material tothe thickness of the sublayer of metallic material of each successivelayer of the plurality of layers gradually changes from the saidradiation surface to the oscillator device.
 2. The transducer of claim1, wherein said ratio gradually increases from the radiation surface tothe oscillator device.
 3. The transducer of claim 1, wherein said ratiogradually decreases from the radiation surface to the oscillator device.4. The transducer of claim 1, wherein said oscillator device includes anacoustic damper.
 5. The transducer of claim 1, wherein said plasticmaterial is selected from the group consisting of mylar, polester andpolyolefine; and wherein said metallic material is selected from thegroup consisting of copper, chromium, nickel, iron, coblat, oxides ofthe foregoing metals, and nitrides of the foregoing metals.